.square, .circle {
    width: 100px;
    height: 100px;
    display: inline-block;
    float: left;
}
.circle {
    border-radius:50%;
}
#blockRed {
    background-color: red;
    position: relative
}
#blockYellow {
    background-color:yellow;
    position: relative
}
#container {
    background-color: darkblue;
    margin: 0 auto;
    width: 400px;
    height: 400px;
}
#blockWood {
    background-image: url(https://www.photos-public-domain.com/wp-content/uploads/2018/08/wood-grain-texture.jpg);
}